Volcanoes are vents in the Earth's surface from which molten rock, debris, and steam issue.
About 1,900 volcanoes are active today or known to have been active in historical times.
Almost 90 percent of volcanoes are in the Ring of Fire, a band of volcanoes circling the edges of the Pacific Ocean.
Most volcanoes occur at plate boundaries, areas where huge slabs of rock meet in the Earth's lithosphere, or outer shell.
Volcanoes can rise in subduction zones, areas where plates meet and one is pushed beneath another. Molten rock rises to the surface and forms a volcano.
Volcanoes can also arise in spreading centers, or rifts, where plates move away from each other, spreading or splitting the Earth's crust.
Intraplate volcanoes are caused by hot spots deep within the Earth. Magma rises and erupts as lava through cracks in the Earth's surface, forming volcanoes.
An eruption begins when magma, the molten rock from deep in the Earth's crust, rises toward the surface.
Volcanoes can erupt in a combination of ways: explosively with hard pyroclastic material; explosively with fluid lava (lava fountains); effusively with hard pyroclastic flows (clouds of ash and gases); and effusively with fluid lava.
Although some volcanoes are considered extinct, almost any volcano is capable of rumbling to life again.
Volcanoes provide valuable mineral deposits, fertile soils, and geothermal energy.